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We can get to Mars in 3 days, . . .sort of, maybe. In this episode of SciShow Space Reid Reimers explains the possibilities of photonic propulsion in use with space travel.

Hosted by: Reid Reimers
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There's been all kinds of buzz lately about some new space tech that could send a spacecraft to Mars in three days and maybe even get spaceships to exoplanets that are light-years away, which kind of sounds like something out of a science fiction novel, but a group of researchers from the University of California-Santa Barbara is working on a new way to travel in space. Known as photonic propulsion, it would use a giant set of lasers to push ships along, and if it works, it could eventually be used to explore other star systems. But that's a big if.

The project is called DEEP-IN, and its goal is to use electromagnetic acceleration to get ships close-ish to the speed of light, fast enough that interstellar travel could actually make sense. These days, our spaceships use chemical acceleration, in other words, they burn fuel, and they are fast. They're just not fast enough to travel to other stars in any reasonable amount of time, which is where electromagnetic acceleration comes in. Instead of using chemical energy to push itself forward, a DEEP-IN spacecraft would use the energy from electromagnetic radiation, more specifically, the energy from a huge set of lasers powered by sunlight.

That laser array technology is a whole project all by itself called the Directed Energy System for Targeting of Asteroids and Exploration, or DE-STAR, and being developed by the same team of researchers, and as you might have noticed from the first part of that acronym, DE-STAR would be a laser system with multiple uses, like, destructive uses. DE-STAR is mainly being designed to protect Earth, both by diverting asteroids that are heading for us, and by vaporizing space debris. This photon-driven propulsion thing is just a bonus, and a pretty huge one.

It's based on the idea that light has a lot of pushing power, which comes from its momentum. We might not feel that push from just walking around on Earth, but a giant reflector screen in space does feel it. That's the science behind solar sails, and we've already built spaceships that use them. When the photons in light bump into a solar sail, their momentum is transferred, and the spacecraft is propelled forward a little bit. DE-STAR's lasers would provide lots of light that we could use to take the solar sails concept one step further and start building spacecraft that use laser sails.

See, solar sails are limited, because the light coming from the sun only comes with so many photons, but this laser sail will have concentrated beam of photons shooting directly at it. It's the difference between a sprinkler and a fire hydrant. When it gets hit by that laser beam, the laser sail and anything that happens to be attached to it, like a spaceship, is going to start zooming through space. A ship using a laser sail wouldn't have to carry as much fuel, which would mean that it could have a much lower mass. It could also, in theory, go very, very fast. With a huge laser array putting out 50-70 gigawatts of power, a 100 kilogram ship about the size of Voyager 1 could travel at around 1.5% of the speed of light, nearly 300 times Voyager's top speed.

But, not surprisingly, there are still challenges to solve when it comes to making spaceships powered by giant lasers. We could send a smaller probe to Mars in three days, or a larger craft on a trip that would take about a month, but we'd need a giant square laser array that's 10 kilometers long on each side, which presents some obvious problems. Getting stuff to space is expensive, let alone 100 square kilometers worth of high-powered laser equipment, and even if we get everything to the right spot, it would be incredibly difficult to assemble.

Building a laser sail for this super fast trip to Mars would also be tough. According to researchers, it would have to be only a micron thick - that's a thousandth of a millimeter. But to work properly, that whisper thin laser sail would have to weigh about as much as the spaceship itself. Meaning that it would have to be a huge thin sail, but strong enough to be stable while the ship is moving ridiculously fast, and we also have no idea how to slow the spacecraft down once it gets to wherever it's going. So if we're talking about sending ships to Mars in three days, we're probably getting ahead of ourselves a little bit here.

Really, the first step to making this research a reality is building a much smaller set of lasers. The team's plan, assuming NASA chooses to move forward with their idea, is to start with a laser array that's only one meter square, and then we just keep building bigger sets of lasers, 'til we eventually figure out how to build one that's ten kilometers on a side, then we can use that to launch what are known as wafer sats. These miniature spacecraft would weigh no more than a gram, but they'd have sensors, a power source, teeny tiny thrusters, and communications equipment. That giant set of lasers could accelerate those wafer sats to about 25% of the speed of light, sending them light-years away, where they could tell us about interstellar space and exoplanets. They might even be able to reach the nearby star system, Alpha Centauri only 15 years after they launch.

So there's still a huge amount of research, technology development, and testing needed before we can use lasers to propel any sort of spacecraft, even a little wafer one, and it's going to be a very long time before we're zooming over to Mars in just a few days, but it probably is possible.

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